Write database pages in sorted order to reduce file fragmentation.

This is an experimental change.  It is not clear if it will help any.
It may well be removed in the future. (CVS 3255)

FossilOrigin-Name: fe3e70a7275d68acb6fb8ea5d62bed3e9d8d2766

manifest

@@ -1,5 +1,5 @@
-C Asserts\sadded\sthat\sverify\sthat\sthe\scode\sworks\scorrectly\nthat\sticket\s#1849\sclaims\sis\swrong.\s(CVS\s3254)
+C Write\sdatabase\spages\sin\ssorted\sorder\sto\sreduce\sfile\sfragmentation.\nThis\sis\san\sexperimental\schange.\s\sIt\sis\snot\sclear\sif\sit\swill\shelp\sany.\nIt\smay\swell\sbe\sremoved\sin\sthe\sfuture.\s(CVS\s3255)
-D 2006-06-15T13:22:23
+D 2006-06-15T14:31:07
 F Makefile.in 200f6dc376ecfd9b01e5359c4e0c10c02f649b34
 F Makefile.linux-gcc 2d8574d1ba75f129aba2019f0b959db380a90935
 F README 9c4e2d6706bdcc3efdd773ce752a8cdab4f90028
@@ -62,7 +62,7 @@ F src/os_unix.c 17d91581a0ab478a06cb6f257b707a4c4a93e5a7
 F src/os_unix.h 5768d56d28240d3fe4537fac08cc85e4fb52279e
 F src/os_win.c c6976ae50b61fb5b7dce399e578aa1865f02b84f
 F src/os_win.h 41a946bea10f61c158ce8645e7646b29d44f122b
-F src/pager.c ddd05666bb89808a516baef2c186d6a75887ae90
+F src/pager.c 7ef4c5098b378d81d3687919d9de99ea99bbb6bb
 F src/pager.h 43f32f3847421f7502cfbb66f4eb2302b8033818
 F src/parse.y 14ce6fb228b5a06fd5c10999aed2cbbfafe630a7
 F src/pragma.c 27d5e395c5d950931c7ac4fe610e7c2993e2fa55
@@ -366,7 +366,7 @@ F www/tclsqlite.tcl bb0d1357328a42b1993d78573e587c6dcbc964b9
 F www/vdbe.tcl 87a31ace769f20d3627a64fa1fade7fed47b90d0
 F www/version3.tcl 890248cf7b70e60c383b0e84d77d5132b3ead42b
 F www/whentouse.tcl 97e2b5cd296f7d8057e11f44427dea8a4c2db513
-P 6380a9b118cf972c3c7d4886ecdb62c44f2208ca
+P d145dc1c4bce403ec967bfd0024002d4de57f833
-R 7c8c83f41ad732828e19404b0580f1d2
+R 2239e05327023592b50f3f9abbe32c42
 U drh
-Z 7d97f3dfe7f27a791bb719d9b2679d1a
+Z 91f8e7798ae68e6bd7062239e69b6453

manifest.uuid

@@ -1 +1 @@
-d145dc1c4bce403ec967bfd0024002d4de57f833
+fe3e70a7275d68acb6fb8ea5d62bed3e9d8d2766

src/pager.c

@@ -18,7 +18,7 @@
 ** file simultaneously, or one process from reading the database while
 ** another is writing.
 **
-** @(#) $Id: pager.c,v 1.268 2006/05/07 17:49:39 drh Exp $
+** @(#) $Id: pager.c,v 1.269 2006/06/15 14:31:07 drh Exp $
 */
 #ifndef SQLITE_OMIT_DISKIO
 #include "sqliteInt.h"
@@ -161,7 +161,7 @@ struct PgHdr {
   u8 needSync;                   /* Sync journal before writing this page */
   u8 alwaysRollback;             /* Disable dont_rollback() for this page */
   short int nRef;                /* Number of users of this page */
-  PgHdr *pDirty, *pPrevDirty;    /* Dirty pages sorted by PgHdr.pgno */
+  PgHdr *pDirty, *pPrevDirty;    /* Dirty pages */
   u32 notUsed;                   /* Buffer space */
 #ifdef SQLITE_CHECK_PAGES
   u32 pageHash;
@@ -2260,6 +2260,68 @@ static int syncJournal(Pager *pPager){
   return rc;
 }
 
+/*
+** Merge two lists of pages connected by pDirty and in pgno order.
+** Do not both fixing the pPrevDirty pointers.
+*/
+static PgHdr *merge_pagelist(PgHdr *pA, PgHdr *pB){
+  PgHdr result, *pTail;
+  pTail = &result;
+  while( pA && pB ){
+    if( pA->pgno<pB->pgno ){
+      pTail->pDirty = pA;
+      pTail = pA;
+      pA = pA->pDirty;
+    }else{
+      pTail->pDirty = pB;
+      pTail = pB;
+      pB = pB->pDirty;
+    }
+  }
+  if( pA ){
+    pTail->pDirty = pA;
+  }else if( pB ){
+    pTail->pDirty = pB;
+  }else{
+    pTail->pDirty = 0;
+  }
+  return result.pDirty;
+}
+
+/*
+** Sort the list of pages in accending order by pgno.  Pages are
+** connected by pDirty pointers.  The pPrevDirty pointers are
+** corrupted by this sort.
+*/
+#define N_SORT_BUCKET 25
+static PgHdr *sort_pagelist(PgHdr *pIn){
+  PgHdr *a[N_SORT_BUCKET], *p;
+  int i;
+  memset(a, 0, sizeof(a));
+  while( pIn ){
+    p = pIn;
+    pIn = p->pDirty;
+    p->pDirty = 0;
+    for(i=0; i<N_SORT_BUCKET-1; i++){
+      if( a[i]==0 ){
+        a[i] = p;
+        break;
+      }else{
+        p = merge_pagelist(a[i], p);
+        a[i] = 0;
+      }
+    }
+    if( i==N_SORT_BUCKET-1 ){
+      a[i] = merge_pagelist(a[i], p);
+    }
+  }
+  p = a[0];
+  for(i=1; i<N_SORT_BUCKET; i++){
+    p = merge_pagelist(p, a[i]);
+  }
+  return p;
+}
+
 /*
 ** Given a list of pages (connected by the PgHdr.pDirty pointer) write
 ** every one of those pages out to the database file and mark them all
@@ -2293,6 +2355,7 @@ static int pager_write_pagelist(PgHdr *pList){
     return rc;
   }
 
+  pList = sort_pagelist(pList);
   while( pList ){
     assert( pList->dirty );
     rc = sqlite3OsSeek(pPager->fd, (pList->pgno-1)*(i64)pPager->pageSize);